1. Field of the Invention
This invention relates to a differential test for distinguishing strains of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) from one another and for differentiating field stains from the currently used vaccine strain.
Porcine reproductive and respiratory syndrome (PRRS) was first reported in North America in 1987 (Keffaber, 1989) and emerged in Europe in late 1990 (Wensvoort et al., 1991). Recently, PRRS has gained world-wide attention because of its economic impact on the swine industry. The disease is characterized by reproductive failure in pregnant sows and respiratory problems in pigs of all ages (Loula, 1991). The causative agent of PRRS is a small (50-60 nm) positive-stranded RNA enveloped virus. The genome is a polyadenylated RNA molecule of about 15 kb and contains eight open reading frames (ORFs). Viral proteins are expressed by six subgenomic mRNAs which are transcribed from the negative strand using a body sequence derived from the 5' end of the viral genome (Meulenberg et al., 1995). The virus replicase is most likely encoded by ORFs 1a/1b by a -1 frameshift (Conzelmann et al., 1993). Open reading frames 2 to 4 encode for putative structural proteins (Van Nieuwstadt, 1995). ORF 5 encodes for a envelope (E) glycoprotein of approximately 25 kDa. A non-glycosylated membrane (M) protein of 18 kDa is encoded by ORF 6 and the nucleocapsid (N) protein of 15 kDa is encoded by ORF 7 (Meulenberg et al., 1995; Conzelmann et al., 1993).
Morphologically and morphogenetically, PRRSV resembles equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV) (Conzelmann et al., 1993; Meulenberg et al., 1993; Plagemann and Moennig, 1991; Snijder and Spaan, 1995). As a result of common features shared by these viruses, they have been tentatively grouped into a new virus family, Arteriviridae (Conzelmann et al., 1993; Meulenberg et al., 1993, Plagemann and Moennig, 1992).
2. Description of the Prior Art
Although the clinical features of PRRS in the United States and in Europe appear similar, several recent studies have indicated phenotypic, antigenic, and genetic differences exist among PRRSV isolates (Bautista et al., 1993; Meng et al., 1994; Wensvoort et al., 1992). The amino acid sequences of ORF 2 through ORF 7 of North American isolates share only 55-79% homology with those of European descent (Meulenberg et al., 1995). It is believed that the glycosylated structural envelope protein E, encoded by ORF 5, is partially responsible for these serological variations among isolates of PRRSV (Meulenberg et al., 1995). Protein E is the counterpart of G.sub.L of EAV (de Vries et al., 1992) and VP-3 of LDV (Gogeny et al., 1993). Both (G.sub.L and E) proteins contain a large internal hydrophobic region which has been thought to anchor these proteins in the membrane (Meulenberg et al., 1995). The existence of neutralizing epitopes in VP-3 of LDV (Harty and Plagemann, 1988) and G.sub.L of EAV (Balasuriya et al., 1995) was demonstrated using monoclonal antibodies and sequencing escape mutants. More recent studies indicate that protein E plays a role in inducing neutralizing antibodies (Persch et al., 1995).
A modified-live-virus vaccine strain RespPRRS.RTM. of PRRSV is currently used in the United States for the prevention of the respiratory facet of the syndrome. Although the vaccine strain is attenuated, it, like virulent field strains of PRRSV, has the ability to persist for at least several weeks in a vaccinated pig. Consequently, the source of PRRSV isolated from diagnostic samples is sometimes in question and it would be highly desirable to have a positive assay for identifying the source.